1. Field of the Invention
This invention relates to an oximetric sensor that is nonadhesively attached to a human digit (e.g., a finger) via an inexpensive, disposable finger cot to analyze the blood of a patient by calculating the concentration of blood constituents (e.g., the saturation level of oxygen in the patient's blood) while minimizing potentially interfering noise artifact signals.
2. Description of Related Art
Electromagnetic energy is often transmitted through or reflected from a medium to determine the characteristics of the medium. In the medical field, optical energy can be transmitted or reflected through human tissue and subsequently measured to determine information about the tissue rather than extracting material from a patient's body for testing. This form of noninvasive measurement can be performed quickly and easily, and has proven to be more comfortable to the patient.
Noninvasive physiological monitoring of body functions is often required when treating a patient. For instance, the available supply of oxygen in the body (i.e., blood oxygenation) is often monitored during surgery. Today this measurement commonly is performed by a noninvasive technique that measures the ratio of incident to transmitted (or reflected) light through a blood-perfused portion of the body, such as, for example, a finger, an ear lobe, or the forehead.
Transmission of optical energy as it passes through the body is strongly affected by several factors. Such factors include the thickness of the tissue through which the energy passes, optical coupling, the optical angle, and the distance between the detector and the source of energy (i.e., the optical path length).
Several parts of the human body are soft and compressible, and therefore are ideally suited to transmit optical energy. For example, a human digit, such as the finger, comprises skin, muscle, tissue, bone, blood, etc. Although the bone is relatively incompressible, the tissue surrounding the bone is easily compressed when an external pressure is applied to the finger. Accordingly, such digits are well suited for the transillumination or transreflection of optical energy for blood monitoring purposes.
Optical probes have been used in the past for both invasive and noninvasive applications. In the typical optical probe, a light emitting diode (LED) is placed on one side of the human tissue while a photodetector is placed on the opposite side. Such conventional optical probes are primarily useful when a patient is relatively motionless and in environments which are characterized by low ambient light.
By way of particular example, one well-known noninvasive measuring device in which an optical probe is used in health applications is the pulse oximeter which measures the pulse rate and the percent of oxygen available in an arterial vessel. Up until the early 1980s, clinicians relied upon arterial blood gas analysis to evaluate gas exchange and oxygen transport within the human body. Although the arterial blood gas test gives valuable information, it only reflects a patient's oxygenation status for one moment in time. On the other hand, pulse oximetry permits a continuous, noninvasive measurement of a patient's arterial oxygen saturation status.
Oximetry is based on the principal that blood hemoglobin absorbs red and infrared light differently when carrying oxygen, in the form of oxyhemoglobin, than when not carrying oxygen, in the form of reduced hemoglobin. Prior oximetric sensors sense the absorption rate of the optical energy by the blood hemoglobin to determine arterial blood oxygen saturation; i.e., the amount of hemoglobin-carried oxygen in relation to the total hemoglobin-carrying capacity.
For this reason, prior oximetric sensor commonly includes a photodetector and a pair of LEDs which emit both red and infrared light. The sensor is packaged in such a way that the LEDs and photodetector are placed on opposite sides of a vascular bed which, in the transillumination case, is usually a finger, ear lobe, or toe. In the reflectance case, the LEDs and the photodetector are placed side by side over a vascular bed, usually the forehead, but separated by a barrier which blocks light from reaching the detector without first passing through the tissue sample. When properly positioned, the LEDs emit known wavelengths of both red and infrared light for transmission or transreflection through the vascular bed for receipt by the detector.
The photodetector produces a signal in response to unabsorbed light passed through the vascular bed to the detector. This signal conventionally is converted to digital form and then supplied to a computer or microprocessor which computes the ratio of red light to infrared light absorption. The absorption data is then utilized to determine the arterial blood oxygen saturation values which then may be displayed on a monitor or a strip chart. Because the light that is directed into the vascular bed also is at least partially absorbed by the nearby tissue and bone material, the oximeter typically utilizes the alternating bright and dim signals caused by arterial pulsations to further clarify the presence of both reduced hemoglobin and oxyhemoglobin, as known in the art.
Prior pulse oximeters provide health care providers with the ability to assess second to second changes in a patient's arterial oxygen saturation. This enables possible intervention before hypoxemia occurs. (Hypoxemia results from lack of oxygen in the blood which can lead to brain damage or even death.) The health care provider also is able to evaluate the patient's response to treatment on a continuous basis.
Initially utilized in the operating room, pulse oximetry is becoming increasingly common in other parts of the hospital, including emergency rooms, adult and neonatal intensive care units, and post anesthesia care units. It is expected that pulse oximeters will also find their way into the general ward and even outside the hospital by medical emergency technicians and private physicians. It is in these new areas that the prior optical probes have proven to be inadequate due to patient movement and the relatively noisy environments in which they are used.
One conventional optical sensor that is adhesively attached to a patient's finger is disclosed in U.S. Pat. No. 4,830,014, issued May 16, 1989, to Goodman et al. In its pre-application configuration, the sensor has a planar "I" shape with an adhesive layer covering an entire side. The area of the sensor which is intended to cover the curved surface of the finger is narrowed so as to provide less stability at the fingertip. The probe includes a complex, layered structure formed by a plurality of juxtaposed layers joined together by interposed layers of adhesive. A first layer includes apertures through which a light source and an optical detector communicate with each other. Another layer firmly engages (i.e., adheres to) the patient's finger. The sensor consequently moves with the finger. The configuration of this sensor also demands that the light source and the detector must be aligned precisely with the corresponding apertures to insure that light will pass through the apertures and between the source and the detector.
Prior optical sensors, such as that disclosed by Goodman, suffer from several drawbacks. For instance, such sensors which adhesively attach to the patient's skin, are susceptible to decoupling when the patient's finger is moved erratically. That is, the occasional movement of the finger commonly places the skin on one side of the finger in tension and places the skin on the opposite side of the finger in compression. The source and detector, which are attached to the skin on opposites sides of the finger, thus are moved as the skin is placed in tension or compression, and such movement can result in misalignment between the source and the detector and/or can change the radiation angle between the source and detector. The movement can also change the optical path length between the source and the detector. As a result, the misalignment and/or increase in the path length produce motion artifact signals, which are unpredictable and thus uncompensatable, and which consequently causes the output signal from the detector to be difficult to interpret and not representative of the amount of the transilluminated or transreflected light.
Another known optical sensor is described in U.S. Pat. No. 5,125,403 issued Jun. 30, 1992 to Culp. A woven tube which is folded partially inside itself secures a side-folding light source and detector structure about a patient's finger tip. The finger engages the side-folding structure and pushes it inside the woven tube causing the tube to begin sliding inside out. However, the woven tube is unstable, tending to reverse its inside out movement. Moreover, the side-folding structure can slide off the tip of the finger thereby requiring the entire assembly to be refolded and refitted onto the finger. Flexing the finger can also cause disengagement, and the woven structure does not sufficiently act to straighten the finger after the finger has been flexed.